A Pyridyl-1,2-azaborine Ligand for Phosphorescent Neutral Iridium(III) Complexes

A novel 1,2-azaborine (i.e., 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene, 1a) has been synthesized and used for the first time as a B–N alternative to common cyclometalating ligands to obtain neutral phosphorescent iridium(III) complexes (i.e., 2a, 3, and 4) of general formula [Ir(C∧N)2(N∧NB)], where C∧N indicates three different cyclometalating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline). Moreover, the azaborine-based complex 2a was compared to the isoelectronic C=C iridium(III) complex 2b, obtained using the corresponding 2-(naphthalen-2-yl)pyridine ligand 1b. Due to the dual cyclometalation mode of such C=C ligand, the isomeric complex 2c was also obtained. All new compounds have been fully characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and the X-ray structure of 2a was determined. The electronic properties of both ligands and complexes were investigated by electrochemical, density functional theory (DFT), and photophysical methods showing that, compared to the naphthalene analogues, the azaborine ligand induces a larger band gap in the corresponding complexes, resulting in increased redox gap (basically because of the highest occupied molecular orbital (HOMO) stabilization) and blue-shifted emission bands (e.g., λmax = 523 vs 577 nm for 2avs2b, in acetonitrile solution at 298 K). On the other hand, the 3LC nature of the emitting state is the same in all complexes and remains centered on the pyridyl-borazaronaphthalene or its C=C pyridyl-naphthalene analogue. As a consequence, the quantum yields of such azaborine-based complexes are comparable to those of the more classical C=C counterparts (e.g., photoluminescence quantum yield (PLQY) = 16 vs 22% for 2avs2b, in acetonitrile solution at 298 K) but with enhanced excited-state energy. This proves that such type of azaborine ligands can be effectively used for the development of novel classes of photoactive transition-metal complexes for light-emitting devices or photocatalytic applications.


■ INTRODUCTION
The introduction of heteroatoms in carbon-based chemical structures is a popular strategy to expand chemical diversity. In particular, borazaro compounds (also called azaborines), 1 have raised great interest in recent years for their potential applications in biomedical research 2 and optoelectronics. 3−6 The peculiarity of this class of compounds consists in the replacement of one aromatic sp 2 C�C unit with an isoelectronic B−N one, 7 leading to a wide range of possible organic structures with different physical and chemical properties. 8−11 Both theoretical calculations and experimental data prove that the aromaticity and thermal stability of azaborines are only slightly lower than those of the corresponding C�C aromatic systems. 12 Despite the same number of valence electrons and a similar stability, the polarization of the B−N bond in azaborines modifies their chemical reactivity and molecular properties, with respect to the corresponding "all-carbon" analogues. Indeed, this local dipole moment can significantly alter the character of the frontier molecular orbitals (FMOs) and the intermolecular interactions in the solid phase. 13,14 In fact, the delocalized π orbitals of an aromatic system can interact with the nitrogen-donor and boron−acceptor orbitals, decreasing the highest occupied molecular orbital (HOMO)− lowest unoccupied molecular orbital (LUMO) gap, while preserving molecular planarity. 15 Such characteristics make most azaborines luminescent molecules, 16,17 with promising applications in organic light-emitting diodes (OLEDs). 18,19 In general, iridium-based complexes are widely used in solid-state lighting devices, 20,21 as probes for in vitro bioimaging, 22−25 and in photoredox catalysis. 26,27 At present, the leading emissive materials in electroluminescent devices (such as OLEDs and light-emitting electrochemical cells (LECs)) are transition-metal complexes (TMCs), due to their relatively facile synthesis and their emission from triplet excited states, which allows for greater electroluminescence efficiency compared to singlet emitters. 28−31 Among all TMCs, cyclometalated iridium(III) complexes have proven to be the most versatile, mainly due to their tunable phosphorescent emission, covering the whole visible spectrum from blue to red, together with high photoluminescence quantum yields and good chemical stability. 32 Actually, both neutral and ionic (anionic and cationic) luminescent iridium(III) complexes can be synthesized, using chelating ligands, which could be monoanionic, dianionic, and neutral. The neutral complexes are the most suitable for OLED applications since they allow the device preparation via vacuum sublimation. 33,34 Herein, we focus our attention on a novel class of bidentate ligands for transition-metal complexes, based on functionalized 1,2-azaborines. In particular, we decided to functionalize the boron atom of a 2,1-borazaronaphthalene with a pyridyl moiety, to obtain a suitable chelating ligand for iridium(III) complexes.
Molander and co-workers reported an effective methodology for the direct synthesis of 2,1-borazaronaphthalenes containing different N-heterocyclic substituents bound to boron, via the reaction of substituted 2-aminostyrenes with organotrifluoroborates. 35 However, no compounds carrying the pyridin-2-yl substituent were reported, probably due to the instability of the pyridyl trifluoroborate used in the proposed reaction method. 36 The functionalization of 2,1-borazaronaphthalenes with a pyridin-2-yl unit is of great interest since it could create new chelating ligands able to effectively coordinate a metal center through the two nitrogen atoms (i.e., one from the pyridyl moiety and the other from the azaborine unit), forming the typical five-membered metallacycle, as shown in Chart 1. Since 1,2-azaborines are aromatic compounds, the deprotonation of the nitrogen of the borazaronaphthalene moiety and the resulting binding to the iridium(III) ion could be thought of as a cyclometallation reaction with the anionic azaborine ligand serving as a strong field chelator. This could significantly alter the relative energy of the frontier molecular orbitals of the octahedral complexes, providing novel properties to the corresponding luminescent materials (see below).
In addition, to evaluate the effects of this new B−N ligand on the photophysical properties of the related complexes, we also synthesized and characterized the equivalent isoelectronic C�C ligand 1b and its isosteric iridium(III) complex 2b (Chart 1), along with its isomer 2c, which is unavoidably formed during the synthesis. ■ EXPERIMENTAL SECTION General Information. 2-Isopropenylaniline (5), 2-bromopyridine (7), and 2-naphthylboronic acid (9) were commercially available. 2-(Naphthalen-2-yl)pyridine (1b) was prepared according to reported methods. 37 Analytical-grade solvents and all of the commercially available reagents were used without any further purification, unless otherwise specified. Chromatographic purifications were performed using 230−400 mesh silica gel (pore size 60 Å) and activated basic Al 2 O 3 (58 Å, 60 mesh powder). Tetrahydrofuran (THF) and toluene have been dried before use by distillation from Na/benzophenone. Reactions that needed anhydrous conditions were performed under dried nitrogen flow (inert atmosphere). The glassware used in these reactions was placed in an oven at +70°C for at least 3 h immediately before use. 1 The yellowish solid formed dissolved to give a  pale-yellow solution when the mixture is refluxed. The mixture was  stirred for 1 h and then was cooled to room temperature and the  excess BCl 3 was removed under vacuum together with the solvent.  The crude was dissolved in anhydrous THF, and then the solvent was  evaporated again under vacuum to remove via stripping the BCl 3 traces. Product 6 was used in the next step without further purification. In a round-bottom flask, 2-bromopyridine 7 (500 μL, 5.25 mmol) was dissolved in dry THF under nitrogen atmosphere. The solution was then cooled down to −78°C, and n-butyl lithium and 1.6 M hexane solution (5.9 mL) was added dropwise to get an orange solution containing 2-lithiumpyridine 8 that was used for the azaborine synthesis, without any additional purification. Product 6 was dissolved in 2 mL of anhydrous THF and was added dropwise to the solution of 2-lithiumpyridine 8 at −78°C; then, the mixture was left to reach room temperature and stirred overnight. The solvent was then removed in vacuum, and the resulting crude was purified on silica gel (hexane/ethyl acetate 9:1). Product 1a was obtained as a yellow-brown solid (500 mg, 65% yield). 1  General Procedure for the Synthesis of Complexes 2a, 3, and 4. The desired iridium(III) dimer 10, 11, or 12 (0.03 mmol) was dissolved in a 3:1 mixture of CH 2 Cl 2 /EtOH (6 mL); then, AgPF 6 (3 equiv) was added in the absence of light. The mixture was kept in the dark and stirred at 60°C overnight. The solution was evaporated to dryness, and the resulting solid was used in the next step of the synthesis without further purification. A solution of compound 1a (2.5 equiv, 16.5 mg, 0.075 mmol) and NaH 60% dispersion in mineral oil (6 equiv, 7.2 mg, 0.18 mmol) in anhydrous CH 2 Cl 2 (5 mL) was stirred for 1 h at room temperature and then added dropwise to the previous solid, and the mixture was stirred for further 24 h at room temperature. After this time, the solvent was evaporated and the crude was purified by flash chromatography.    General Procedure for the Synthesis of Complexes 2b and 2c. Iridium(III) dimer 10 (150 mg, 0.14 mmol) was dissolved in 2ethoxyethanol (2 mL); then, ligand 1b (115 mg, 4 equiv) and K 2 CO 3 (155 mg, 8 equiv) were added. The mixture was degassed with three vacuum−nitrogen cycles and stirred under a nitrogen atmosphere in the dark at 90°C for 36 h. The solution was evaporated to dryness, and the resulting solid was purified by flash chromatography on silica deactivated with triethylamine using a mixture of petroleum ether/ ethylic ether/dichloromethane 4.5:0.5:0.3. Electrochemistry. Voltammetric experiments were performed using a Metrohm AutoLab PGSTAT 302N electrochemical workstation in combination with the NOVA 2.0 software package. All of the measurements were carried out at room temperature in acetonitrile solutions with a sample concentration of ≈0.5 mM and using 0.1 M tetrabutylammonium hexafluorophosphate (electrochemical grade, TBAPF 6 ) as the supporting electrolyte. Oxygen was removed from the solutions by bubbling nitrogen. All of the experiments were carried out using a three-electrode setup (BioLogic VC-4 cell, volume range: 1−3 mL) using a glassy carbon working electrode (having an active surface disk of 1.6 mm in diameter), the Ag/AgNO 3 redox couple (0.01 M in acetonitrile, with 0.1 M TBAClO 4 supporting electrolyte) as the reference electrode, and a platinum wire as the counter electrode. At the end of each measurement, ferrocene was added as the internal reference. Osteryoung square-wave voltammograms (OSWV) were recorded with a scan rate of 125 mV s −1 , an SW amplitude of ±20 mV, and a frequency of 25 Hz. Cyclic voltammograms (CV) were recorded at 100 mV s −1 .

Inorganic Chemistry
Computational Details. Density functional theory (DFT) calculations were carried out using the B.01 revision of the Gaussian 16 program package 38 in combination with the M06 global-hybrid meta-generalized gradient approximation (meta-GGA) exchange− correlation functional. 39,40 The fully relativistic Stuttgart/Cologne energy-consistent pseudopotential with multielectron fit was used to replace the first 60 inner-core electrons of the iridium metal center (i.e., ECP60MDF), and it was combined with the associated triple-ζ basis set (i.e., cc-pVTZ-PP basis). 41 On the other hand, the Pople 6-31G(d,p) basis was adopted for all other atoms. 42,43 All of the compounds were fully optimized without symmetry constraints, both in the ground state (S 0 ) and in their lowest triplet states (T n ), using the polarizable continuum model (PCM) to simulate acetonitrile solvation effects. 44−46 Frequency calculations were always used to confirm that every stationary point found by geometry optimizations was actually a minimum on the corresponding potential-energy surface (no imaginary frequencies). The structural overlap between the X-ray crystal structure of complex 2a and its ground-state computed one was obtained using the Visual Molecular Dynamics (VMD) program 47 by minimizing the root-mean-square deviation (RMSD) of all of the atomic positions, except hydrogen atoms. Timedependent DFT (TD-DFT) calculations, 48,49 carried out at the same level of theory used for geometry optimizations, were employed to map the excited-state scenario of the investigated molecules in their optimized S 0 geometry. For ligands 1a and 1b, the lowest six singlet excited states were also computed at the STEOM-DLPNO-CCSD level of theory, 50,51 as implemented in ORCA 5.0.3, 52 using the def2-TZVP basis set 53 for all atoms and perturbative CPCM to simulate acetonitrile solvation. Multiwfn 3.8�a Multifunctional Wavefunction Analyzer was used to process ORCA outputs. 54 Natural transition orbital (NTO) transformations were adopted to obtain a clear and compact orbital representation for the electronic transition density matrix in the case of complex multiconfigurational excitations. 55 To investigate the nature of the triplet states, geometry optimizations and frequency calculations were performed at the spin-unrestricted UM06 level of theory, imposing a spin multiplicity of 3; the ground-state minimum-energy geometry was used as initial guess for T 1 , while TD-DFT optimized geometries were taken as input for the UDFT optimization of upper-lying triplets. All of the pictures showing molecular orbitals and spin-density surfaces were created using GaussView 6. 56 Photophysical Measurements. The spectroscopic investigations were carried out in spectrofluorimetric-grade solvents (i.e., dichloromethane and acetonitrile). The absorption spectra were recorded with PerkinElmer Lambda 950 spectrophotometer. For the photoluminescence experiments, the sample solutions were placed in fluorimetric Suprasil quartz cuvettes (10.00 mm) and dissolved oxygen was removed by bubbling argon for 30 min. The uncorrected emission spectra were obtained with an Edinburgh Instruments FLS920 spectrometer equipped with a Peltier-cooled Hamamatsu R928 photomultiplier tube (spectral window: 185−850 nm). An Osram XBO xenon arc lamp (450 W) was used as the excitation light source. The corrected spectra were acquired by means of a calibration curve, obtained using an Ocean Optics deuterium−halogen calibrated lamp (DH-3plus-CAL-EXT). The photoluminescence quantum yields (PLQYs) in solution were obtained from the corrected spectra on a wavelength scale (nm) and measured according to the approach described by Demas and Crosby, 57 using an air-equilibrated water solution of tris-(2,2′-bipyridyl)ruthenium(II) dichloride (PLQY = 0.028) 58 as reference for the complexes and/or an air-equilibrated water solution of quinine sulfate in 1 N H 2 SO 4 (PLQY = 0.546) 59 as reference for the ligands. The emission lifetimes (τ) were measured through the time-correlated single photon counting (TCSPC) technique using a HORIBA Jobin Yvon IBH FluoroHub controlling a spectrometer equipped with a pulsed NanoLED (λ exc = 280 and 370 nm) or pulsed SpectraLED (λ exc = 370 nm) as the excitation source and a red-sensitive Hamamatsu R-3237-01 PMT (185−850 nm) as the detector. The analysis of the luminescence decay profiles was accomplished with the DAS6 Decay Analysis Software provided by the manufacturer, and the quality of the fit was assessed with the χ 2 value close to unity and with the residuals regularly distributed along the time axis. To record the 77 K luminescence spectra, samples were put in quartz tubes (2 mm inner diameter) and inserted into a special quartz Dewar flask filled with liquid nitrogen. Poly(methyl methacrylate) (PMMA) films containing 1% (w/w) of the complex were obtained by drop-casting, and the thickness of the films was not controlled. Solid-state PLQY values were calculated by corrected emission spectra obtained from an Edinburgh FLS920 spectrometer equipped with a barium sulfate-coated integrating sphere (diameter of 4 in) following the procedure described by Wurth et al. 60 Experimental uncertainties are estimated to be ±8% for τ determinations, ±10% for PLQYs, and ±2 and ±5 nm for absorption and emission peaks, respectively.  [1,2]azaborinine 1a was synthesized by means of a modification of Dewar's procedure, 13,61 as represented in Scheme 1. The 1,2-azaborine aromatic ring was obtained through a direct cyclization of 2-isopropenylaniline 5 with BCl 3 to generate the intermediate 6 that can undergo nucleophilic substitution at the reactive B−Cl unit. 2-Lithiumpyridine 8 was chosen as a nucleophile. Owing to its poor stability, it was prepared in situ by the reaction of 2bromopyridine 7 with n-butyl lithium, under inert nitrogen atmosphere, and immediately reacted with 6 (Scheme 1).
The reaction mixture was stirred at room temperature for 24 h. Eventually, the crude was purified by column chromatography on basic alumina. Finally, precipitation from a hexane/ CH 2 Cl 2 solution gave pure complexes 2a, 3, and 4 in good yields (38−61%), which were fully characterized by NMR spectroscopy and mass spectrometry (see the Experimental Section and Figures S4−S13). Although the synthesis of neutral iridium(III) complexes by means of cyclometallation reactions with 2-phenylpyridine derivatives usually requires high temperatures, our methodology avoids such drastic conditions, which the azaborine ligand could not withstand. 65 With isosteric ligand 1b in hand, we synthesized the corresponding complexes 2b and 2c by a modification of the classical methods reported in the literature (Scheme 2). 65  matter of fact, ligand 1b can undergo cyclometallation in two different positions (i.e., at C-1 and at C-3); therefore, a mixture of complexes is expected. By tuning reaction conditions, it was possible to obtain both isomers and we found that by heating dimer 10 and 1b in 2-ethoxyethanol at +90°C for 36 h under an inert atmosphere, a 1:1 mixture of tris-cyclometalated isomers 2b and 2c was obtained. The neutral complexes can be separated through accurate column chromatography on deactivated silica gel to give 2b and 2c in 32 and 43% yields, respectively. It must be emphasized that, due to the less sterically hindered structure, 2c is the most thermodynamically stable and 2b slowly converts into the most stable structural isomer when subjected to more drastic reaction conditions (higher temperature) during the synthesis. Even in this case, they were fully characterized by NMR spectroscopy and mass spectrometry (see the Experimental Section and Figures S14− S17). X-ray Characterization. Single crystals of complex 2a were obtained by slow diffusion of ethyl ether in dichloromethane solution. The compound crystallizes in the orthorhombic Pbcn space group. The structure showed the arrangement of the three ligands around a pseudo-octahedral iridium ion (Figure 1 and Tables S1−S8). The two nitrogen atoms of the 2-phenylpyridine ligands occupy the two apical positions in a trans arrangement, as in other cyclometalated iridium(III) complexes reported in the literature, 21,32,33 with N−Ir bond lengths of 2.055 and 2.030 Å. On the other hand, the two remaining adjacent equatorial positions are filled with the two nitrogen of the azaborine ligand; in this case, the N−Ir bonds are longer: 2.184 Å for the azaborine moiety and 2.160 Å for the pyridine one.
The B−N bond length is 1.398 Å, which is slightly shorter than a full-organic azaborine (i.e., 1.427 Å). 13 The azaborine bite angle (i.e., N−Ir−N angle) is 78.43°, which is comparable to that of the two phenylpyridine ligands (i.e., 80.1 and 79.6°). On the contrary, unlike the phenylpyridines, the azaborine ligand itself is slightly bent (mean distance from ligand leastsquares plane: 0.11 vs 0.06 Å) and the N−Ir−N−B−C fivemembered ring is not planar, with the iridium atom 0.62 Å out of the least-squares plane passing through the ligand. Conversely, the ancillary ligand itself is expected to be planar, corroborating the aromatic character of the borazaronaphthalene moiety (see below). 18 DFT Calculations: Ground-State Properties. For a better understanding of the electronic and optical properties of both ligands 1a and 1b and the related iridium(III) complexes, DFT and TD-DFT calculations were carried out using the M06 hybrid meta-GGA exchange−correlation functional, 39,40 together with the polarizable continuum model (PCM) to take into account acetonitrile solvation effects. 44−46 The validity of the adopted computational method has already been effectively tested on similar systems (as demonstrated by several publications on the topic), 27,32 and it is also proved by the nearly superimposable geometries of the available X-ray structure of 2a and its theoretically optimized one ( Figure S18).
As far as the ligands are concerned, the main structural difference between such compounds is the unusual full planarity of the azaborine ligand (1a), which is estimated to exist merely as a single conformer with the two nitrogen atoms in cis-arrangement ( Figure S19). On the contrary, the naphthyl-pyridine analogue (1b) is nonplanar, as commonly expected for other phenylpyridine derivatives, 68 showing two nearly isoenergetic conformers with a dihedral angle of ±20°b etween its two aromatic rings ( Figure S19). Figures S20 and S21 show the energy diagrams and the frontier molecular

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pubs.acs.org/IC Article orbitals of both ligands (considering different conformers), also evaluating the effect of the methyl substituent, which is present in the azaborine ligand 1a, but not in 1b. The HOMO−LUMO gap is wider in 1a, due to both HOMO stabilization and LUMO destabilization (i.e., 4.96 vs 4.77 eV for 1a vs 1b, respectively�Figure S20). No significant differences were appreciated by comparing the two nearly isoenergetic 1b conformers (i.e., 1b A or 1b B �Figure S21). On the contrary, the presence of the methyl substituent on the naphthyl ring of 1b (as in the theoretical ligand 1b′) leads to a 0.10 eV destabilization of the HOMO, which is mainly located on such polycyclic moiety, leaving the LUMO virtually unperturbed ( Figure S21); taking the methyl effect into consideration, our results suggest that the replacement of the C�C bond with the isoelectronic B−N one is potentially able to stabilize the HOMO by more than 0.2 eV. Concerning the iridium(III) complexes, the energy diagrams and the frontier molecular orbitals (FMOs) of the azaborinebased complexes 2a, 3, and 4 are reported in Figure 2, together with those of the isoelectronic C�C analogues 2b and 2c. As done for the ligands, also the methylated analogue of complex 2b was investigated (i.e., 2b′, see Figure S22). As already observed for 1b vs 1b′, only the destabilization of the occupied orbital on the naphthyl ring is observed upon methylation, but such an orbital is the HOMO−1 in the complexes so that the HOMO−LUMO gaps is virtually identical in both 2b and 2b′.
Comparing all of the synthesized complexes, a rather complicated scenario is observed due to a tricky orbital interplay within sets of nearby occupied and unoccupied FMOs. In the case of azaborine-based complexes, the HOMO is always predominantly centered on the borazaronaphthalene ligand and the HOMO−1 is mainly located on the iridium d orbitals and on the phenyl moieties of the cyclometalating ligands. On the contrary, the scenario is reversed when the azaborine ligand is replaced with its C�C analogue (as in 2b and 2c, Figure 2). Such an interplay between this couple of orbitals is due to the different energy of the orbital centered on the iridium ion and on the phenyl moieties of the cyclometalating ligands, while the highest π orbital located on the ancillary ligand (i.e., the borazaronaphthalen-or naphthalen-pyridine) remains energetically unaffected along all of the series at −5.49 ± 0.05 eV. Indeed, the former orbital is highly stabilized in complex 3 (by 0.31 eV, if compared to 2a) due to the presence of the electron-withdrawing fluorine substituents, 32 while it is destabilized by approx. 0.2 eV in 2b and 2c, due to the stronger field exerted by the cyclometalating C�C ancillary ligand. Notably, in the case of 2a, such pairs of orbitals are virtually isoenergetic.
As far as unoccupied FMOs are concerned, analogous sets of three nearby π* orbitals can be identified along all of the complexes of the series (Figure 2). For azaborine-based derivatives 2a and 3, LUMO and LUMO+1 are located on each of the pyridine moieties of the two ppy-based ligands, while LUMO+2 is accommodated on the pyridine part of the azaborine ligand. Only a minor overall stabilization of this set of orbitals is observed by passing from 2a to 3 (approx. 0.08 eV), due to the effect of the electron-withdrawing fluorine substituents. 32 Notably, although the same orbital order is also preserved in 4, a remarkable LUMO and LUMO+1 stabilization of ≈0.97 eV (compared to 2a) is observed due to the replacement of the pyridine moiety on the cyclometalating ligands with the π-extended quinoxaline one. On the contrary, the stabilization on LUMO+2 is only 0.11 eV since centered on the same azaborine ligand.
If comparing 2a with its isoelectronic C�C analogues 2b and 2c, a progressive destabilization of the π* orbitals centered on the ppy ligands is observed, with a concomitant stabilization of those located on the azaborine-based ligand (in 2a) or its C�C analogue (in 2b and 2c). As a result, the LUMO remains centered on the pyridine moiety of the ppy ligand in 2a and 2b, but it is located at lower energy and on the naphthyl-pyridine ligand in 2c.
Electrochemistry. The scenario envisaged by DFT calculations is substantially confirmed by electrochemical experiments, carried out using square-wave and cyclic voltammetry in acetonitrile solution at ambient temperature (see the Experimental Section for further details). Selected electrochemical data are summarized in Table 1, whereas the square-wave voltammograms of ligands 1a and 1b and of all of the investigated complexes are reported in Figures S23 and 3, respectively.
None of the redox processes exhibited by the borazaronaphthalene free ligand 1a or by its corresponding complexes (2a, 3, and 4) is reversible, as also found for other similar azaborine systems. 69 On the contrary, full electrochemical reversibility is observed for all of the first oxidation and reduction processes

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pubs.acs.org/IC Article occurring in the isoelectronic C�C ligand 1b and in the associated complexes 2b and 2c, as commonly observed in cyclometalated iridium(III) complexes ( Figure S24). 32,33 The oxidation potentials of all three azaborine-based complexes are comparable, with nearly identical values for 2a and 4 (i.e., +0.46 and +0.50 V, respectively) and a 0.1 V anodically shifted potential for 3, due to electron-withdrawing fluorine substituents. Such experimental evidences are in excellent agreement with DFT predictions, expecting virtually identical HOMO topologies for all of the three complexes and a 0.10 eV stabilization in 3, compared to 2a (see above). On the other hand, the oxidation of the two C�C isoelectronic complexes (2b and 2c) occurs at much lower values compared to the 2a analogue, with the oxidation potential of 2b being slightly more positive than that of 2c (i.e., +0.25 vs +0.23 V, respectively). Again, the correlation with DFT estimates is impressive: (i) the calculated HOMO destabilization in 2b and 2c, with respect to 2a, is just slightly underestimated if compared to the corresponding cathodic shift in their oxidation potential (i.e., 0.17 eV vs 0.22 V); (ii) the higher oxidation potential of 2b, if compared to 2c, is perfectly justified by the 0.02 eV more stabilized HOMO of the former complex.
As far as reductions are concerned, as predicted by DFT, far less negative potentials are observed for complex 4, due to the presence of the easily reducible π-extended quinoxaline moiety on each of the two cyclometalating ligands; indeed, two reduction processes are observed at −1.71 and −1.96 V (Figure 3). At more negative potentials, the two reduction peaks of complexes 2a and 3 are detected, which are attributable to the reduction of the pyridine moiety on each of the two cyclometalating ligands, in analogy to 4. As normally observed for fluorinated complexes, 21,32 the two peaks of complex 3 are shifted to more positive values by 0.10 eV compared to fluorine-free analogue 2a, in line with DFT calculations (Figure 2).
On the other hand, the replacement of the azaborine ancillary ligand with the C�C counterpart does not significantly alter the reduction potential of 2b and only a minor cathodic shift of 0.03 V is observed compared to 2a (Table 1); this can be easily rationalized by considering that the first reduction is expected to involve the same pyridine moiety of the ppy ligand, which is identical in both 2b and 2a. On the contrary, the different chelation mode of the C�C ancillary ligand in 2c results in a more positive reduction potential by 0.10 V, compared to 2b (Table 1). This is because such reduction now involves the highly stabilized π* orbital of the ancillary ligand itself and not the ppy ligand ( Figure 2). All of these results are perfectly caught by DFT calculations, which estimate a 0.02 eV LUMO destabilization by passing from 2a to 2b and a 0.11 eV LUMO stabilization if 2b is compared to 2c (Table 1 and Figure 2).
Photophysical Characterization of the Ligands. The photophysical properties of the azaborine free ligand 1a were investigated in room-temperature solution and in rigid matrix at 77 K, and compared to that of the C�C isoelectronic analogue 1b. The room-temperature absorption and emission spectra of 1a and 1b in acetonitrile solution are reported in Figure 4, together with fluorescence and phosphorescence spectra in the rigid matrix at 77 K. The absorption and emission spectra of both ligands are only marginally affected by the solvent polarity, and only a minor red-shift of the absorption profiles is observed by passing from more polar acetonitrile solutions to dichloromethane ( Figure S25).
The absorption profile of the azaborine ligand 1a strongly differs from the one of 1b, mainly for the lack of the intense absorption band present for the C�C analogue and centered at approx. 250 nm. In addition, the lowest-energy S 0 → S 1 transition of 1a is vibronically structured and rather intense (ε = 9.6 × 10 3 M −1 cm −1 ) with maximum at 320 nm ( Figure 4); on the contrary, 1b displays a similar transition in the same spectral region but with a much weaker profile (ε ≈ 6 × 10 2 M −1 cm −1 ), which is partially covered by a more intense unstructured band centered at 293 nm ( Figure 4).
Surprisingly, TD-DFT calculations estimate S 0 → S 1 excitations with considerable HOMO → LUMO character and high oscillator strengths for both 1a and 1b (i.e., f = 0.405 and 0.208, Table S9), which is in contrast to the experimental absorption spectrum of 1b. Indeed, more advanced STEOM-DLPNO-CCSD calculations reveal that the adopted TD-DFT method fails to predict the correct excited-state order in 1b (compare Tables S9 and S10). In the case of the azaborine ligand 1a, both methods agree that the population of S 1 occurs by an intense π−π* HOMO → LUMO transition having a minor charge-transfer nature from the borazaronaphthalene moiety to the pyridine ring; such vertical excitation is estimated at 303 nm by TD-DFT approach and at 310 nm for the CCSD-based calculation (Tables S9 and S10). Moreover, if vibronic effects are taken into account, the agreement between the lowest-energy absorption band of the experimental spectrum of 1a in acetonitrile and the corresponding TD-DFT vibronically resolved S 0 → S 1 transition is remarkable ( Figure S26). Indeed, the computed absorption band is underestimated by approx. 0.05 eV and the vibronic progression is nicely reproduced. The highly structured profile observed for such a lowest-energy band is due to the very similar geometry adopted by 1a in both S 0 and S 1 ( Figure S27). In addition, both TD-DFT and STEOM-DLPNO-CCSD methods agree in assigning the S 0 → S 2 excitation to a weak n−π* transition (f = 0.004) involving the pyridine moiety.
On the other hand, in the C�C isoelectronic analogue 1b, there are two lowest-lying excited states with π−π* character (with different charge-transfer contributions) and the n−π* transition involving the pyridine moiety is the S 0 → S 3

Inorganic Chemistry
pubs.acs.org/IC Article excitation. Anyway, only STEOM-DLPNO-CCSD is able to correctly predict that the S 0 → S 1 transition is weakly absorbing (f = 0.001 at 318 nm) and the S 0 → S 2 is the bright one (f = 0.316 at 281 nm); such states are energetically reversed by TD-DFT (compare Tables S9 and S10), leading to the incorrect prediction of the absorption spectrum of 1b. A possible explanation for such failure can be ascribed to the well-known underestimation of excitation energies that TD-DFT methods suffer in the case of charge-transfer transitions. 70 The lack of vibronic progressions in the absorption spectrum of 1b can be tentatively ascribed to pronounced structural rearrangements upon excitation. Indeed, 1b is not planar in the ground state ( Figure S19), while planarization is expected for all of the lowest excitations due to the population of a C�C bonding orbital between the naphthalene and pyridine rings (Tables S9 and S10). Despite the differences in their absorption profiles, 1a and 1b ligands present very similar fluorescence spectra, with rather structured emission profiles centered at approx. 360 nm, regardless of solvent polarity (Figures 4 and S25 and Table 2). Even fluorescence quantum yields are virtually identical for both 1a and 1b (i.e., around 20 and 25% in acetonitrile and dichloromethane solutions, respectively), despite the azaborine ligand 1a displaying a 10 times shorter excited-state lifetime with respect to the C�C isoelectronic analogue 1b (i.e., τ ≈ 1 vs 10 ns, Table 2).
The 77 K emission spectra of both 1a and 1b in butyronitrile frozen glass show the presence of fluorescence and phosphorescence ( Figure 4). For both ligands, the fluorescence spectrum at 77 K exhibits just a more vibrationally resolved structure, but it is virtually superimposable with the one at room temperature. If 1a and 1b display an S 1 state with comparable energy, this is not the case for their lowest triplet: the phosphorescence spectrum of the azaborine ligand 1a is considerably blue-shifted with respect to the one of its C�C analogue 1b (Figure 4). Indeed, unrestricted DFT calculations confirm the different nature of such triplets, despite both displaying a fully planar geometry, as observed for their lowest singlets. In the case of 1a, T 1 is fully centered on the borazaronaphthalene moiety, while, for the triplet of 1b, the spin-density distribution is also spread on the pyridine ring ( Figure S28). This is reflected in the different phosphorescence lifetime of these molecules (i.e., 2.60 vs 0.90 s for 1a vs 1b, see Table S11) and in a drastically dissimilar vibronic progression of the two phosphorescence spectra, which are nicely reproduced by vibronically resolved DFT calculations ( Figure S29).
Photophysical Characterization of the Complexes. All of the iridium(III) complexes were studied in solution at room temperature (CH 3 CN and CH 2 Cl 2 ), in butyronitrile glass at 77 K and in PMMA matrix at a sample concentration of 1% by weight. The absorption spectra of all of the complexes in acetonitrile and dichloromethane solutions are reported in Figures 5 and S30, respectively; no notable changes are observed by changing the polarity of the solvent.
As typically found in cyclometalated iridium(III) complexes, the main absorption bands in the spectral region between 200 and 300 nm are attributed to spin-allowed ligand-centered (LC) π−π* transitions. At longer wavelengths (300−400 nm), the weaker and broader bands can be assigned to chargetransfer transitions with mixed metal-to-ligand, ligand-toligand, and intraligand charge-transfer (MLCT/LLCT/ ILCT) nature of different multiplicity; 21,32 an exception to this general assignment is the band peaking at 370 nm in the absorption spectrum of complex 4, which is attributed to the quinoxaline moiety. 71 The lowest-energy absorption profiles of all of the complexes are magnified in the inset of Figure 5, and it should be emphasized that such low-energy bands are virtually insensitive to solvent polarity ( Figure S30), suggesting a predominantly ligand-centered nature of the lowest-lying triplet states. The lowest-energy absorption profiles of 2a and 3 are similar, but the latter is blue-shifted due to the electronwithdrawing fluorine substituents on the cyclometalating ligands. 32 On the contrary, as already suggested by DFT calculations and electrochemical measurements, the spectral profiles of the C�C complexes (2b and 2c) extend to longer wavelengths compared to the azaborine-based analogue 2a.
To get a deeper insight into the excited-state properties of all of the complexes, the lowest-lying triplet states were computed by means of TD-DFT methods. Tables S12−S16 illustrate the lowest triplet excitations of 2a−c, 3, and 4, reported as couples of natural transition orbital (NTO). 55 Moreover, a compact representation of triplet-state energy landscape at the Franck− Condon region is shown in Figure 6 for all of the investigated complexes. The most complicated excited-state scenario is expected for complex 2a, due to the presence of three nearly   LC states, respectively, localized on the two nonequivalent ppy ligands and on the azaborine ancillary one (Table S12). In the case of 3, the presence of the fluorine substituents on the cyclometalating ligands increases the energy of the corresponding 3 LC states by 0.16 eV, while keeping the one centered on the azaborine relatively unperturbed (i.e., 2.72 vs 2.70 eV in 2a, Tables S12 and S13). The scenario is reversed in the case of the isoelectronic C�C analogues 2b and 2c, which have the same ppy cyclometalating ligands of 2a: for both complexes, the two nearly degenerate 3 LC states localized on the two ppy moieties remain unperturbed at 2.67 ± 0.02 eV, as in 2a, but a substantial stabilization of the triplet state centered on the C� C ancillary ligand is calculated, and it is expected to be maximized in 2c (Tables S15 and S16). As a consequence, although for different reasons, complexes 2b, 2c, and 3 are all expected to emit from a 3 LC state centered on the ancillary ligand, which is the lowest triplet. On the contrary, the two lowest triplets in 4 are 3 LC states centered on each of the two cyclometalating ligands, which are strongly stabilized with respect to 2a due to the π-extended quinoxaline moiety and lay only 1.97 and 2.05 eV above S 0 (Table S14).
Normalized emission spectra of all of the complexes are reported in Figure 7 in room-temperature acetonitrile and dichloromethane solutions (top), and in butyronitrile glass at 77 K (bottom). The corresponding luminescence properties and photophysical parameters are summarized in Table 3. The emission spectra of the azaborine-based complexes 2a and 3 are nearly superimposable, with emission maxima at 523 and 524 nm in room-temperature acetonitrile solution (Table 3). At 298 K, the emission bands of both complexes are only marginally affected by changing the solvent polarity and exhibit clues of vibronic progression (Figure 7, top). At 77 K, the spectra of both 2a and 3 become strongly structured, but their band onset does not shift compared to room temperature. These experimental findings suggest that the emission arises from a 3 LC state with a virtually identical nature in both complexes. Indeed, spin-unrestricted DFT optimizations carried out on the lowest triplet states of 2a and 3 demonstrate that, after relaxation, the lowest triplet state of both complexes is the 3 LC state centered on the azaborine ligand, which will be ultimately responsible for emission (Figure 8). The adiabatic energy difference between such 3 LC state and S 0 is estimated to be 2.50 and 2.52 eV for 2a and 3, respectively, which is in good agreement with the emission maxima in acetonitrile solution at 298 K (i.e., approx. 2.37 eV, see Table 3).
It should be noted that, for the fluorinated complex 3, the order of the three lowest triplet states (T 1 −T 3 ) is preserved upon relaxation; indeed, the energy gap between the lowest 3 LC state centered on the azaborine ligand (i.e., T 1 ) and the upper-lying ones located on the two dfppy ligands (i.e., T 2 and T 3 ) increases from 0.12 (at the Franck−Condon region, Table S13) to 0.22 eV (as adiabatic energy difference after triplet relaxation, Figure 8).
On the other hand, a state flipping is observed in the case of 2a, having nearly isoenergetic T 1 −T 3 at the Franck−Condon region (Table S12). Due to a larger geometrical relaxation of the 3 LC state centered on the azaborine ligand (i.e., T 3 ), this triplet becomes 0.05 eV lower in energy compared to the ones located on the two ppy ligands (i.e., T 1 and T 2 , Figure 8). Actually, due to the small energy gap between these three states, a thermal equilibration could not be ruled out and this can possibly explain the higher photoluminescence quantum yield of 2a compared to 3 (i.e., approx. 20 vs 6%, Table 3), with faster radiative decay constants (i.e., k r ≈ 1 × 10 5 vs 3 × 10 4 for 2a and 3, respectively).
In the case of complex 4, its room-temperature emission band shows a broad and unstructured profile, and it is strongly red-shifted compared to 2a and 3. Notably, such red-shift is more pronounced as the solvent polarity increases (i.e., λ em = 672 vs 692 nm in dichloromethane vs acetonitrile solution). Moreover, a rigidochromic shift is also observed in frozen

Inorganic Chemistry
pubs.acs.org/IC Article matrix at 77 K (i.e., λ em = 638 nm, Table 3). Such experimental findings suggest an emission from a triplet state with a high charge-transfer character, but the relatively long excited-state lifetime at 77 K (i.e., 10.7 μs, Table 3) suggests a more ligandcentered emission. Indeed, a clear attribution of the emitting state is difficult since the three lowest triplet states of 4 are calculated to be isoenergetic after relaxation (i.e., ΔE < 5 meV); most probably, a thermal equilibrium between T 1 and T 3 exists at room temperature, while the 3 LC state (i.e., T 2 ) can be predominant at 77 K, due to the lack of solvent reorganization supporting the full stabilization of the other two triplets with more charge-transfer character. Last but not least, the emission properties of the azaborinebased complex 2a can be compared with that of the two isoelectronic C�C analogues (2b and 2c). U-DFT calculations indicate that, upon full geometrical relaxation, the lowest triplet state of 2b and 2c remains T 1 ( Figure S31), so emission is expected to arise from such 3 LC state centered on the naphthalene−pyridine ancillary ligand in both complexes. Such a state is estimated to be located at 2.29 and 2.32 eV above S 0 for 2b and 2c, respectively (to be compared to 2.50 eV for 2a, Figure 8). Indeed, a red-shift of 0.18 eV is experimentally observed in the emission profiles 2b and 2c, if compared to 2a (Figure 7). The photophysical properties of the two C�C isomers are very similar and lower photoluminescence quantum yields are only observed for 2c, basically due to faster nonradiative deactivation pathways (e.g., PLQY = 38.8 vs 8.4% for 2b vs 2c in oxygen-free CH 2 Cl 2 solutions at 298 K, Table 3).
The emission properties of the complexes were also investigated in the solid state, that is, dispersed in a poly(methyl methacrylate) (PMMA) matrix at a sample concentration of 1% by weight. The corresponding emission spectra are reported in Figure S32, and the photophysical parameters are gathered in Table 4.
A strong enhancement of the luminescence is observed for all of the complexes once dispersed in PMMA (i.e., PLQY = 22−67%, Table 4). Except for the complicated case of 4 (where a complex interplay between nearby triplets is expected, see above), for all other complexes, the enhancement in PLQYs is basically due to a strong reduction of the k nr

■ CONCLUSIONS
Neutral luminescent iridium(III) complexes carrying a 1,2azaborine as B−N alternatives to commonly used C�C cyclometalating ligand have been obtained for the first time. An efficient procedure for the synthesis of 2-(pyridin-2-yl)-2,1borazaronaphthalene 1a has been set up, and this novel compound has been exploited as a ligand to introduce a new class of neutral phosphorescent iridium(III) complexes equipped with different cyclometalating ligands. Such complexes (2a, 3, and 4) have been obtained in satisfactory yields and fully characterized. In addition, stereoisomeric complexes (2b and 2c) containing the isoelectronic C�C cyclometalating 2-(pyridin-2-yl)-naphthalene ligand 1b have been synthesized and compared with the analogue 2a. The aromatic character of the ligand is maintained despite the B−N bond, and its rigidity seems to effectively limit distortions and nonradiative deactivation processes of the excited state. The azaborine ligand 1a displays similar luminescence properties if compared with the isoelectronic C�C analogue 1b (e.g., λ max = 359 vs 362 nm, PLQY = 22 vs 19% in acetonitrile solution at 298 K); however, they display very different absorption profiles and excited-state lifetimes (e.g., τ = 1 vs 10 ns). The neutral azaborine complexes exhibit good luminescence properties in solution (PLQY up to 19% for 2a), which are further enhanced in PMMA matrix (PLQY = 22− 69%). Except for 4 (which is equipped with highly π-extended cyclometalating ligands), the emission of the azaborine-based complexes arises from 3 LC states centered on the azaborine ligand itself. The comparison of the electronic properties of 2a with the two analogous complexes bearing the C�C ligand 1b showed that the presence of the B−N polarized bond induces a larger band gap than in 2b or 2c, resulting in increased redox gap (due to HOMO stabilization) and to blue-shifted emission bands; however, the nature of the emitting state is preserved. The photoluminescence quantum yields and lifetimes of the azaborine-based complex 2a are comparable with those of the standard C�C counterparts, proving the effectiveness of this approach.
This work introduces a new class of luminescent iridium(III) complexes, which might be expanded by suitably designed 1,2azaborines to vary the luminescence properties and potential applications thereof.
NMR spectra of ligands 1 and complexes 2−4; crystal data of compound 2a; supporting figures and tables, including square-wave voltammograms of ligands 1 and complexes 2a and 2b; DFT calculations; and photophysics for ligands 1 and complexes 2−4 (PDF)